1. Field of the Invention
This invention relates to a semiconductor controlled rectifier which is turned on by gate current supplied from a gate electrode.
2. Description of the Prior Art
A semiconductor controlled rectifier which is turned on by gate current comprises generally a semiconductor substrate of pnpn structure consisting of four continuous layers of alternately different conductivity types, a pair of main electrodes in ohmic contact with the exposed surface of the outer p-type and n-type layers respectively, and a gate electrode in contact with the exposed surface portion of one of the intermediate layers. The four-layer region between the main electrodes placed in the non-conducting state in rendered conducting and load current starts to flow in such a semiconductor controlled rectifier when gate voltage is applied across the gate electrode and the main electrode in contact with the outer layer adjacent to the intermediate layer in contact with the gate electrode for causing a flow of gate current across these two electrodes in a state in which bias voltage for reversely biasing the pn junction formed between the intermediate layers is applied across the opposite main electrodes. The semiconductor controlled rectifier is said to be turned on when the semiconductor controlled rectifier in such a non-conducting state is rendered conducting.
The turn-on mechanism of the semiconductor controlled rectifier which is turned on by the gate current supplied from the gate electrode is such that a small area of the four-layer region in the vicinity of the gate electrode is initially turned on in response to the supply of the gate current and this turned-on area is increased with the lapse of time until the entire region is turned on. Therefore, when the inrush current increase rate di/dt during turning on the semiconductor controlled rectifier is considerably large, the density of load current in the limited area of the four-layer region in the vicinity of the gate electrode (the small area portion initially turned on) is excessively large and the temperature of this region is unusually increased to such an extent that the semiconductor controlled rectifier may be finally subject to thermal breakdown.
Various methods have been proposed heretofore in an effort to prevent this objectionable thermal breakdown by increasing the capability of the semiconductor controlled rectifier to withstand a large inrush current increase rate di/dt. For example, a proposal has been made in which the gate electrode of a semiconductor controlled rectifier is formed in an annular shape so that initial conduction can take place at the entire periphery of the four-layer region opposite to the annular gate electrode. However, this proposal is defective in that an excessively large gate current is required for turning on the semiconductor controlled rectifier.
The most desirable condition for a semiconductor controlled rectifier is that a widest possible area of the semiconductor controlled rectifier can be quickly turned on with a small gate power. A semiconductor controlled rectifier of an amplifying gate type is known as one of those which satisfy the condition above described. Such a semiconductor controlled rectifier is disclosed in, for example, U.S. Pat. No. 3,526,815 (Swedish Pat. No. 311,701) and comprises a semiconductor substrate consisting of four continuous layers of pnpn structure, the outer n-type layer of said semiconductor substrate including a main region and an auxiliary region which is isolated from the main region by a portion of the adjacent intermediate layer and has an area smaller than that of the main region, a pair of main electrodes in ohmic contact with the surface of the outer p-type layer and the surface of the main region of the n-type layer respectively, a gate electrode in contact with the portion of the intermediate layer adjacent to the auxiliary region and remote from the main region, and an auxiliary electrode connecting the surface of the auxiliary region with the intermediate layer. The turn-on mechanism of this semiconductor controlled rectifier is such that the four-layer region whose end layer is the auxiliary region, or an auxiliary thyristor is initially turned on by gate current supplied from the gate electrode and load current flowing due to the turn-on of the auxiliary thyristor is used as gate current for turning on the four-layer region whose end layer is the main region, or a main thyristor. Such a semiconductor controlled rectifier can be quickly turned on over a wide area with small gate current. However, this semiconductor controlled rectifier is not still completely satisfactory. More precisely, due to the fact that the turn-on area of the main thyristor is determined by the length of the auxiliary electrode opposite to the main region, the gate current supplied to the main thyristor, that is, the load current flowing due to the turn-on of the auxiliary thyristor must be increased with the increase in the length of the auxiliary electrode opposite to the main region, and in order to increase the load current flowing due to the turn-on of the auxiliary thyristor, the gate current supplied from the gate electrode must be increased to widen the turn-on area of the auxiliary thyristor. Thus, this prior art semiconductor controlled rectifier is defective in that the gate current must be increased in order to increase the capability to withstand a large inrush current increase rate di/dt during turn-on of the rectifier although it is small compared with that required in the case of the annular gate electrode.